Phosphor for light sources and associated light source

ABSTRACT

A phosphor for light sources, the emission from which lies in the short-wave optical spectral region, as a garnet structure A 3 B 5 O 12 . It is activated with Ce, the second component B representing at least one of the elements Al and Ga, and the first component A is terbium or terbium together with at least one of the elements Y, Gd, La and/or Lu.  
     In a preferred embodiment, a phosphor having a garnet of structure (Tb 1-x-y RE x Ce y ) 3 (Al,Ga) 5 O 12 , where RE=Y, Gd, La and/or Lu; 0≦x≦0.5-y; 0&lt;y&lt;0.1 is used.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of copending application Ser. No.10/687,436, filed Oct. 16, 2003, which is a continuation of applicationSerial No. Ser. No. 09/787,208, filed Mar. 15, 2001, now U.S. Pat. No.6,669,866.

TECHNICAL FIELD

The invention relates in particular to a yellow-emitting garnet phosphorfor excitation by a light source with short wavelengths in the visibleblue spectral region, with the result that white light is generated. Alamp (primarily a fluorescent lamp) or an LED (light-emitting diode) isparticularly suitable as the light source.

PRIOR ART

WO 98/05078 has already disclosed a phosphor for light sources and anassociated light source. In that document, the phosphor used is a garnetof the structure A₃B₅O₁₂, the host lattice of which, as first componentA, comprises at least one of the rare earths Y, Lu, Sc, La, Gd or Sm.Furthermore, one of the elements Al, Ga or In is used for the secondcomponent B. The only dopant used is Ce.

A very similar phosphor is known from WO 97/50132. The dopant used inthat document is either Ce or Tb. While Ce emits in the yellow spectralregion, the emission from Tb is in the green spectral region. In bothcases, the complementary color principle (blue-emitting light source andyellow-emitting phosphor) is used to achieve a white luminous color.

Finally, EP-A 124 175 describes a fluorescent lamp which, in addition toa mercury fill, contains a plurality of phosphors. These are excited byUV radiation (254 nm) or also by short-wave radiation at 460 nm. Threephosphors are selected in such a way that they add up to form white(color mixture).

SUMMARY OF THE INVENTION

According to the invention, for light sources from which the emissionlies in the short-wave optical spectral region, a phosphor which has agarnet structure A₃B₅O₁₂ and which is doped with Ce is used, the secondcomponent B representing at least one of the elements Al and Ga and thefirst component A containing terbium. Surprisingly, it has been foundthat under particular circumstances, namely under blue excitation in therange from 420 to 490 nm, terbium (Tb) is suitable as a constituent ofthe host lattice (first component of the garnet) for a yellow-emittingphosphor, the activator of which is cerium. Previously, in this contextTb has only been considered as an activator or coactivator, togetherwith cerium, for emission in the green region, if excitation is producedby cathode rays (electrons) or short-wave UV photons (GB-A 1 600 492 andEP-A 208 713).

In this case, terbium, as the principal constituent of the firstcomponent A of the garnet, can be used on its own or together with atleast one of the rare earths Y, Gd, La and/or Lu.

At least one of the elements Al or Ga is used as the second component.The second component B may additionally contain In. The activator iscerium. In a particularly preferred embodiment, a garnet of thestructure(Tb_(1-x-y)RE_(x)Ce_(y))₃(Al, Ga)₅O₁₂, where

-   RE=Y, Gd, La and/or Lu;-   0≦x≦0.5-y;-   0<y<0.1 is used.

The phosphor absorbs in the range from 420 to 490 nm and can thus beexcited by the radiation from a blue light source, which is inparticular the radiation source for a lamp or LED. Good results havebeen achieved with a blue LED whose emission peak was at 430 to 470 nm.The emission peak of the Tb-garnet: Ce phosphor is at approximately 550nm.

This phosphor is particularly useful for use in a white LED based on thecombination of a blue LED with the Tb-garnet-containing phosphor, whichis excited by absorption of part of the emission from the blue LED andthe emission from which supplements a remaining radiation from the LED,to form white light.

A Ga(In)N-LED is particularly suitable as the blue LED, but any otherroute for producing a blue LED which emits in the range from 420 to 490nm is also suitable. 430 to 470 nm is particularly recommended as theprincipal emission region, since this is where efficiency is highest.

By selecting the type and quantity of rare earths, it is possible tofine-tune the location of the absorption and emission bands, in asimilar way to that which is known from the literature for otherphosphors of type YAG:Ce. In conjunction with light-emitting diodes, itis particularly suitable for x to be 0.25≦x≦0.5-y.

The particularly preferred range for y is 0.02<y<0.06.

The phosphor according to the invention is also suitable for combinationwith other phosphors.

A garnet of structure(Tb_(x)RE_(1-x-y)Ce_(y))₃(Al,Ga)₅O₁₂,

-   where RE=Y, Gd, La and/or Lu;-   0≦x≦0.02, in particular x=0.01;-   0<y<0.1 has proven particularly suitable as the phosphor. Y    frequently lies in the range from 0.01 to 0.05.

Generally, relatively small amounts of Tb in the host lattice serveprimarily to improve the properties of known cerium-activated phosphors,while the addition of relatively large amounts of Tb can be used in acontrolled way in particular to shift the wavelength of the emissionfrom known cerium-activated phosphors. Therefore, a high proportion ofTb is particularly suitable for white LEDs with a low color temperatureof below 5000 K.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is to be explained in more detail below with reference toa number of exemplary embodiments. In the drawing:

FIG. 1 shows an emission spectrum of a first Tb-garnet phosphor;

FIG. 2 shows the reflectance spectrum of this Tb-garnet phosphor;

FIG. 3 shows emission spectra of further Tb-garnet phosphors;

FIG. 4 shows reflectance spectra of the Tb-garnet phosphors from FIG. 3;

FIG. 5 shows emission spectra for further Tb-garnet phosphors;

FIG. 6 shows reflectance spectra for the Tb-garnet phosphors from FIG.5;

FIG. 7 shows an emission spectrum for a white LED with Tb-garnetphosphor.

DETAILED DESCRIPTION OF THE INVENTION Exemplary Embodiment No. 1

The components  9.82 g Yttrium oxide Y₂O₃  2.07 g Cerium oxide CeO₂37.57 g Terbium oxide Tb₄O₇ 26.41 g Aluminum oxide Al₂O₃  0.15 g Bariumfluoride BaF₂ 0.077 g Boric acid H₃BO₃are mixed and comminuted together for two hours in a 250 ml polyethylenewide-necked bottle using 150 g of aluminum oxide balls with a diameterof 10 mm. Barium fluoride and boric acid serve as fluxes. The mixture isfired for three hours in a covered corundum crucible at 1550° C. informing gas (nitrogen containing 2.3% by volume hydrogen) and thenmilled in an automatic mortar mill and screened through a screen with amesh width of 53 um. This is followed by a second firing for three hoursat 1500° C. under forming gas (nitrogen containing 0.5% by volumehydrogen). Then, milling and screening is carried out as after the firstfiring. The phosphor obtained corresponds to the composition(Y_(0.29)Tb_(0.67)Ce_(0.04))₃Al₅O₁₂. It has a strong yellow body color.An emission spectrum for this phosphor when excited at 430 nm and areflectance spectrum for the phosphor between 300 and 800 nm are shownin FIGS. 1 and 2.

Exemplary Embodiment No. 2

The components 43.07 g Terbium oxide Tb₄O₇  1.65 g Cerium oxide CeO₂21.13 g Aluminum oxide Al₂O₃  0.12 g Barium fluoride BaF₂ 0.062 g Boricacid H₃BO₃are intimately mixed and processed as described under Example 1. Thephosphor obtained corresponds to the overall composition(Tb_(0.96)Ce_(0.04))₃Al₅O₁₂ or, in the representation which illustratesthe host lattice, Tb₃Al₅O₁₂:Ce. It has a strong yellow body color. TheX-ray diffraction diagram shows that there is a cubic garnet phase. Theemission spectrum and reflectance spectrum for this phosphor are shownin FIGS. 3 and 4, respectively.

Exemplary Embodiment No. 3

The components 32.18 g Yttrium oxide Y₂O₃  0.56 g Terbium oxide Tb₄O₇ 2.07 g Cerium oxide CeO₂ 26.41 g Aluminum oxide Al₂O₃ 0.077 g Boricacid H₃BO₃are intimately mixed and processed as described under Example No. 1. Thephosphor obtained corresponds to the composition(Y_(0.95)Tb_(0.01)Ce_(0.04))₃Al₅O₁₂. It has a strong yellow body color.The emission spectrum and reflectance spectrum for this phosphor areshown in FIGS. 3 and 4, respectively.

Exemplary Embodiment No. 4

The components 26.76 g Yttrium oxide Y₂O₃  9.53 g Terbium oxide Tb₄O₇ 2.07 g Cerium oxide CeO₂ 26.41 g Aluminum oxide Al₂O₃ 0.149 g Bariumfluoride BaF₂ 0.077 g Boric acid H₃BO₃are intimately mixed and processed as described under Example No. 1. Thephosphor obtained corresponds to the composition(Y_(0.79)Tb_(0.17)Ce_(0.04))₃Al₅O₁₂. It has a strong yellow body color.The emission spectrum and reflectance spectrum for this phosphor areshown in FIGS. 3 and 4, respectively.

Exemplary Embodiment No. 5

The components 30.82 g Yttrium oxide Y₂O₃  0.56 g Terbium oxide Tb₄O₇ 4.13 g Cerium oxide CeO₂ 26.41 g Aluminum oxide Al₂O₃ 0.149 g Bariumfluoride BaF₂ 0.077 g Boric acid H₃BO₃are intimately mixed and processed as described under Example No. 1. Thephosphor obtained corresponds to the composition(Y_(0.91)Tb_(0.01)Ce_(0.08))₃Al₅O₁₂. It has a strong yellow body color.

Exemplary Embodiment No. 6

The components 43.07 g Terbium oxide Tb₄O₇  1.65 g Cerium oxide CeO₂21.13 g Aluminum oxide Al₂O₃ 0.062 g Boric acid H₃BO₃are intimately mixed and processed as described under Example 1, exceptthat the temperature during the two firings is lower by 50° C. in eachcase. The phosphor obtained corresponds to the composition(Tb_(0.96)Ce_(0.04))₃Al₅O₁₂. It has a strong yellow body color. Theemission spectrum and reflectance spectrum for this phosphor are shownin FIGS. 5 and 6, respectively.

Exemplary Embodiment No. 7

The components 43.07 g Terbium oxide Tb₄O₇  1.65 g Cerium oxide CeO₂17.05 g Aluminum oxide Al₂O₃  7.50 g Gallium oxide Ga₂O₃ 0.062 g Boricacid H₃BO₃are intimately mixed and processed as described under Example 1, exceptthat the temperature for the two firings is lower by 50° C. in eachcase. The phosphor obtained corresponds to the composition(Tb_(0.96)Ce_(0.04))Al₄GaO₁₂. It has a strong yellow body color. Theemission spectrum and reflectance spectrum for this phosphor are shownin FIGS. 5 and 6, respectively.

Exemplary Embodiment No. 8

The components 43.07 g Terbium oxide Tb₄O₇  1.65 g Cerium oxide CeO₂12.97 g Aluminum oxide Al₂O₃ 15.00 g Gallium oxide Ga₂O₃ 0.062 g Boricacid H₃BO₃are intimately mixed and processed as described under Example 1, exceptthat the temperature for the two firings is lower by 50° C. in eachcase. The phosphor obtained corresponds to the composition(Tb_(0.96)Ce_(0.04))₃Al₃Ga₂O₁₂. It has a yellow body color. The emissionspectrum and reflectance spectrum of this phosphor are shown in FIGS. 5and 6, respectively.

Exemplary Embodiment No. 9

The components 4.88 kg Yttrium oxide Y₂O₃ 7.05 kg Gadolinium oxide Gd₂O₃161.6 g Terbium oxide Tb₄O₇ 595 g Cerium oxide CeO₂ 7.34 kg Aluminumoxide Al₂O₃ 5.50 g Boric acid H₃BO₃are mixed for 24 hours in a 60 l polyethylene vessel. The mixture isintroduced into crucibles made from aluminum oxide with a capacity ofapprox. 1 l and is fired in a pushed-bat kiln for 6 hours at 1550° C.under forming gas. The fired material is milled in an automatic mortarmill and then finely screened. The phosphor obtained has the composition(Y_(0.50)Gd_(0.45)Tb_(0.01)Ce_(0.04))₃Al₅O₁₂. It has a strong yellowbody color. The emission spectrum and reflectance spectrum for thisphosphor are shown in FIGS. 3 and 4, respectively.

Exemplary Embodiment 10 (LED)

When these phosphors are used in a white LED together with GaInN, astructure similar to that described in WO 97/50132 is employed. By wayof example, identical fractions of phosphor in accordance with Example 1and of phosphor in accordance with Example 4 are dispersed in epoxyresin and a LED with an emission peak of approximately 450 nm (blue) isencapsulated by this resin mixture. The emission spectrum of a white LEDobtained in this way is shown in FIG. 7. In this case, the mixture ofthe blue LED radiation with the yellow phosphor emission results in acolor locus of x=0.359/y=0.350, corresponding to white light of colortemperature 4500 K.

The phosphors described above generally have a yellow body color. Theyemit in the yellow spectral region. When Ga is added or used on its owninstead of Al, the emission shifts more toward green, so that it is alsopossible in particular to achieve higher color temperatures. Inparticular, Ga-containing (or Ga,Al-containing) Tb-garnets and purelyAl-containing Tb-garnets can be used in mixed form in order to be ableto adapt to desired color loci.

1-11. (canceled)
 12. A white light-emitting device comprising: alight-emitting diode that emits blue light; and a phosphor that convertsat least a part of the blue light into longer-wavelength radiation,wherein the phosphor is an Al-containing Tb-garnet phosphor, aGa-containing Tb-garnet phosphor, a Ga,Al-containing Tb-garnet phosphor,or a mixture thereof, whereby the device emits white light.
 13. Thedevice of claim 12 wherein Tb is substituted from 0 to 99 mol % by Y,Gd, La, and/or Lu.
 14. The device of claim 12 wherein less than 50 mol %of Tb is substituted by Y, Gd, La, and/or Lu.
 15. The device of claim 12wherein the phosphor is a yellow-emitting phosphor.
 16. The device ofclaim 12 additionally comprising a yttrium aluminum garnet phosphoractivated with cerium.
 17. The device of claim 12 wherein the blue lightfrom the light-emitting diode has a peak emission in the range from 430to 470 nm.
 18. The device of claim 12 wherein the emission peak of thephosphor is at approximately 550 nm.
 19. The device of claim 12 whereinthe white light has a color temperature below 5000 K.
 20. The device ofclaim 12 wherein the blue light from the light-emitting diode comprisesan excitation in the range from 420 to 490 nm.
 21. The device of claim12 wherein the phosphor is activated by cerium.
 22. A device thatproduces white light, the device comprising: a light-emitting diode thatemits blue light; and a yellow-emitting phosphor, wherein the phosphoris a cerium-activated terbium aluminate that converts at least a part ofthe blue light from the light-emitting diode into yellow light, thedevice emitting a mix of at least blue light and yellow light to producewhite light.
 23. The device of claim 22 wherein at least a portion ofthe aluminum is substituted by gallium.
 24. The device of claim 22wherein a portion of the terbium is substituted by yttrium, gadolinium,lanthanum, and/or lutetium.
 25. The device of claim 23 wherein a portionof the terbium is substituted by yttrium, gadolinium, lanthanum, and/orlutetium.
 26. The device of claim 22 wherein less than 50 mol % of Tb issubstituted by Y, Gd, La, and/or Lu.
 27. The device of claim 23 whereinless than 50 mol % of Tb is substituted by Y, Gd, La, and/or Lu.
 28. Thedevice of claim 22 additionally comprising a yttrium aluminum garnetphosphor activated with cerium.
 29. The device of claim 22 wherein theblue light from the light-emitting diode has a peak emission in therange from 430 to 470 nm.
 30. The device of claim 22 wherein theemission peak of the phosphor is at approximately 550 nm.
 31. The deviceof claim 22 wherein the white light has a color temperature below 5000K.
 32. The device of claim 22 wherein the blue light from thelight-emitting diode comprises an excitation in the range from 420 to490 nm.